It is often desired to cleanse non-ferrous molten metal such as aluminum of impurities prior to a casting process. The impurities include dissolved gas such as hydrogen and/or dissolved metal such as magnesium. The processes of removing dissolved gasses and metals are more commonly referred to as "degassing" and "demagging", respectively.
Methods currently being used by industry for accomplishing demagging and degassing typically include a fluxing process in which a reactive gas, such as chlorine, is mixed with the molten metal. Theoretically, the chlorine gas being more reactive than the molten metal, preferentially combines with the impurities i.e., magnesium, to form light weight compounds that can be easily removed from the molten bath. One such process and apparatus for practicing the process is disclosed in U.S. Pat. No. 4,052,199 and U.S. Pat. No. 4,169,589 both issued to Mangalick and assigned to the Carborundum Company. The patents disclose an apparatus comprising two metallic bath chambers that communicate with each other through a submerged metal transfer conduit. A molten metal pump located in the first chamber pumps molten metal from the first chamber to the second chamber by way of the transfer conduit. A gas injection conduit communicates with the metal transfer conduit and is operative to inject chlorine gas into the molten metal discharged by the pump outlet.
A problem universal to fluxing processes is the potential for uncontrolled escape of chlorine gas or toxic reactants into the atmosphere or working environment. For this reason the rate of chlorine injection into the molten metal must be carefully controlled so that an amount in excess of that which can combine with the impurities is not injected. Apparatus and methods have been suggested which allegedly minimize the escape of raw chlorine gas.
Molten metal pumps are often used to circulate and/or transfer non-ferrous molten materials. One such pump is disclosed in U.S. Pat. No. 2,948,524 issued to Sweeney et. al. and assigned to The Carborundum Company, and apparently forms the pumping means for the gas injection apparatus disclosed in U.S. Pat. No. 4,169,584. The pump comprises a submerged impeller housing that rotatably supports an annular impeller having a plurality of radial ports that communicate a circumferential surface of the impeller with a central bore that defines an inlet to the pump. A platform supporting a driving mechanism is supported above the impeller housing by support posts and the driving mechanism drivingly engages the impeller through a drive shaft that extends from the platform and threadedly engages the impeller. In operation, rotation of the impeller draws molten metal into the inlet from where it travels through the radial bores into the impeller housing, finally being discharged into an outlet conduit.
In the gas injection apparatus disclosed in U.S. Pat. No. 4,169,584, the chlorine gas is injected at the outlet of a Sweeney type pump. This gas injection configuration presents at least two problems. First, the gas is being injected into a fluid under pressure and therefore the injection pressure must exceed the pressure of the fluid. More importantly, the amount of gas discharged will vary with changes in the outlet pressure of the pump. Secondly, in order to provide sufficient contact time with the molten metal, a submerged discharge configuration is required. It would appear that a discharge riser for conveying the molten metal upwardly out of the bath cannot be accommodated. The molten metal must be discharged below the surface of the second bath in order to minimize the potential for gas escape.